The proposed studies build on the findings that interactive T- and B-like lymphocytes are basic features of the adaptive immune system of both jawless and jawed vertebrates, although jawless vertebrates (lampreys and hagfish) generate variable lymphocyte receptors (VLR) for antigen recognition by recombinatorial conversion of incomplete germline VLR genes (VLRA, VLRB and VLRC) into fully assembled VLR genes using diverse leucine-rich-repeat (LRR) donor sequences. Recent studies indicate that VLRA assembly and expression coincides with expression of cytidine deaminase 1 (CDA1) only within the lymphoepithelial thymoid gill region, whereas VLRB and CDA2 expression occurs primarily in hematopoietic tissues. A comprehensive analysis is proposed to elucidate the development, distribution and function of the lamprey VLRA and VLRC lymphocyte lineages in comparison with the B cell-like VLRB lymphocytes to test the hypothesis that the VLRA and VLRC lineages represent agnathan T cell analogues of gnathostome 1/2 and 3/4 T cells with allorecognition responsibility. A long term goal is to identify the lamprey histocompatibility antigens to test the hypothesis that VLRA and VLRC recognize these to achieve self versus non-self discrimination. The first specific aim is to define the distribution, antigen-binding and functional responses of the VLRA +, VLRB+ and VLRC+ lymphocytes and to modulate VLRA and VLRC cell function by treatment with VLR-specific antibodies to gain insight into the function and cooperative potential of the lymphocyte lineages. The second specific aim is to purify the VLRA+, VLRB+ and VLRC+ lymphocyte populations and perform a comparative analysis of each of their transcriptomes to obtain information about the differential expression of cytokines, chemokines, cytokine/chemokine receptors, cytolytic components, VLR co-receptor candidates, co-stimulatory molecules, signaling elements, and transcription factors for the three types of lymphocyte in their quiescent and activated states. The third specific aim is to characterize the allogeneic recognition and response capabilities of VLRA+, VLRB+ and VLRC+ lymphocytes. Pilot in vivo and in vitro studies indicate that the VLRA+ and VLRC+ lymphocytes preferentially respond to allogeneic white blood cells. The stimulatory cell types will be identified, the cellular participants will also be defined for skin allograft rejections, and the modulatory effects of VLRA- and VLRB-specific antibodies will be determined. Gene expression profiles and VLR sequences will be compared for alloantigen-responsive versus non-responsive lymphocytes. VLRB alloantibodies will be induced and VLRA and VLRC receptors from the alloantigen responding cells will be engineered to form secreted chimeric proteins for use in histocompatibility antigen detection. Candidate histocompatibility genes will be examined for genetic polymorphism, expression patterns and immunostimulatory potential.